The present invention relates to a method for restoring a structure based on subsided ground, having at least one crack resulting from the subsidence of the ground.
More particularly, the invention concerns a method for restoring a structure resting on the ground, by treating its foundation soil by injecting an expandable substance in order to close said crack.
A crack appearing on a structure may be a sign of a rupture in the materials constituting the structure and occurs when the stresses caused in the structure concerned exceed the rupture limits from which it was designed.
Some cracks appear because of a sinking of the structure that then gives rise to parasitic stresses, that is to say tensile and/or compression stresses, normally non-existent on the intact structure but which have appeared following the shrinkage of the foundation soil of the structure, and may considerably weaken the structure and thus cause additional weaknesses and disturbances.
The causes of ground subsidence are many. It is generally a case of:                a defect in initial bearing capacity,        accidental decompression of the ground (following a pipe leak or the carrying out of uncontrolled earthwork, etc.).        differential settling.        
Following the phenomenon of settling, the structure undergoes a movement that results in the appearance of cracks in the superstructure as depicted in FIGS. 1, 3 and 5 for example. A part A of the structure follows the settling of the ground on which the structure of the building rests, referred to as the foundation soil and which will be defined in detail subsequently. On the other hand, other parts of the building B, often placed above the first, remain temporarily immobile in an unstable equilibrium, the two parts A, B being separated by cracks. This state of unstable equilibrium creates parasitic stresses that are often detrimental to the life of the structure.
In an ideal case, the works of reinforcement of the foundation soil and of the deeper soil should enable the structure to regain its initial state.
However, the diffusion of stresses in the soil depends on the depth in the soil and the effects of the reinforcement works on the closure of the cracks are also dependent on the depth at which the soil is reinforced.
Boussinesq's formula shows in fact that the stresses are located within a small radius around the vertical to the point of application of the load q for a depth z.
Thus, as depicted in FIG. 10, the diffusion takes place at an angle a with respect to the vertical, the stress sp being uniform inside the diffusion zone over a given horizontal plane.
The value B1 is expressed:B1=B+2*z*tg α
and henceo2=q/(1+2*z/B*tgα)
Generally a value of between 30° and 40° is adopted for α.
FIG. 9 shows schematically the diffusion of σz with the depth from the previous equation.
We observe that, as from a depth of z˜2 B, the value of σz becomes less great in comparison with the loads provided by the structure on the ground. It converges towards zero for a depth of 8 B.
With regard to the settling of the ground, we can establish a direct relationship between the stresses and the deformations. In other words, the maximum settling value is obtained at the base of the foundations and this settling decreases with the depth in order to converge towards zero for a depth of 8 B.
Thus, according to the depth of the soil, two types of soil can be identified:
1. Foundation soil—this comprises a so-called foundation layer Z1 that is situated on the surface, that is to say immediately under the foundations of the structure, and any voids present between the foundation layer and the foundations of the structure. When there is poor bearing capacity, it is liable to suffer settling under the effect of a load and to cause damage in the structure. The foundation soil extends, in accordance with FIG. 3, over a depth of between “0”, that is to say the bottom level of the foundations, and “2 B”, the depth at which the stress σz is around 0.2 σ0, “B” being the width of the foundation element (base or slab) by means of which the load is exerted. The value of “2 B” may therefore for example be between 1.2 m and 2 m.
2. Deep soil—under the foundation layer, a deep layer Z2 is situated at a depth of between 2 B and 8 B. The stress σz there is between 0.2 σ0 and 0.01 σ0 and is therefore negligible. At this depth, a layer of soil, even having poor bearing capacity, suffers less settling because of its relatively great depth vis-à-vis the applied load “q”.
However, for a method for restoring a structure with a view to closing cracks therein to be effective, it is necessary to treat the part of the ground where the stress is maximum, that is to say the foundation soil.
However, it is necessary, before attempting to repair them in order to give the structure its original aspect, to ensure that the deep soil has sufficient bearing capacity.
And if the deep soil requires consolidation, it is necessary first of all to commence the treatment of this layer Z2 with any consolidation method: jet grouting (for injection of grout at high pressure), injection of microcement, or by the method described in European Patent EP 0 851 064 filed by the company Uretek, in accordance with which it is recommended injecting, in the deep soil (layer Z2), an expanding material, while controlling the moment when, following this injection, the building and/or the ground begins to rise.
The methods for restoring a structure having a crack do not therefore include the use of such a method of consolidation at depth.
They are used on the foundation soil including the foundation layer Z1, whereas the soil layer Z2 on which the layer Z1 rests is consolidated at depth, either because it has never suffered stresses that give rise to a loss of bearing capacity, or because it has been redensified following such stresses, or because it has been consolidated by suitable techniques.
For example, the traditional techniques of restoring a structure having a crack envisage, after reinforcement of the deep soil, a reblocking of the cracks after opening, and pinning and then refilling by means of a shrink-free mortar.
These techniques are however not satisfactory for the following reasons:                the part (A) that has suffered the settling remains sunk. Thus the proprietor of the property does not have his structure repaired in the same state as before the damage.        the filling between the part (A) and (B) often comprises a lack of levelling that is not aesthetically satisfactory.        the filling carried out from outside has its limits and does not ensure perfect contact between the elements (A) and (B).        and when the crack is situated on a wall of the structure clad with repetitive patterns, for example a tiled wall, the additional material breaks the repetition of the patterns and degrades the appearance of the wall in question.        
And among the known methods concerning the injection of expanding substance in the foundation soil, the method described in the document U.S. Pat. No. 4,567,708 filed by the company Uretek in 1983 consists of returning a sunken slab to its original planar configuration by injecting an expanding material into the foundation soil. Provision is made from measuring the height of the slab before the additional injection. However, this document is completely silent on the way of closing a crack appearing on a wall supported by the deformed slab.
To overcome these drawbacks the invention proposes a method for reinforcement of the surface foundation soil of a structure, with a quality of finish not yet achieved, the deep soil previously having been reinforced by a suitable technique.